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ISCA Journal of Engineering Sciences _________________________________________ ISCA J. Engineering Sci.

Vol. 1(1), 40-44, July (2012)

International Science Congress Association 40

Effect of Lateral Confinement on Strength of Concrete

Ahmad Khaleek, Yadav R.K.2 and Chandak Rajeev

2

1Structural Engineering, Jabalpur Engineering College, Jabalpur, MP, INDIA

2Department of Civil Engineering, Jabalpur Engineering College, Jabalpur, MP, INDIA

Available online at: www.isca.in Received 22nd June 2012, revised 29th June 2012, accepted 20th July 2012

Abstract

In columns or compression members, lateral reinforcement in the form of hoops, cross-ties, or spirals play an important role

in safeguarding the columns, especially when they are subjected to strong earthquakes or accidental lateral loads. They are

required in any column-whether they are parts of a moment resistant frame or the gravity system in order for them to deform

laterally and provide the required ductility. This paper deals with studies on the compressive strength of concrete specimens

having confinement in various quantities. Studies on reinforced cement concrete specimens were used to understand the

influence of the lateral confinement on the compressive strength of concrete. Cylindrical and square test specimens having

height to diameter (or width) ratio as 2 were used for the study. The specimen was prepared with different spacing of hoops

and having seismic and non-seismic hooks. This paper deals with the variation of peak strength with volumetric ratio of

transverse steel to confined concrete core.

Keywords: Concrete, confinement, reinforced concrete columns, hoops, confined concrete core.

Introduction

It has long been recognized that the strength as well as

deformability of concrete substantially increase wherever

amount of confinement in the form of closed ties (hoops) is

increased. The numerous experimental studies on the role of

confinement Sheikh and Uzumeri1, Saatcioglu and Razvi

2,

Cusson and Paultre3, Mander et.al.

4 have been done in the last

four decades. Sheikh and Uzumer5 proposed a stress-strain

model reflecting the effect of confinement in columns. Mander

et.al6, Saatcioglu and Razvi

7 proposed stress-strain models for

the study of confinement in concrete columns.

The main parameters involved in the confinement are the ratio

of transverse reinforcement i.e. the ratio of volume of hoops to

the volume of confined core of the member, the yield strength of

transverse reinforcement, the compressive strength of concrete,

the spacing of hoops, the longitudinal reinforcement, hoop

pattern.

During earthquakes the failure in columns may occur due to

poor or improper confinements see figure-18. In India and some

other countries it is a practice to tie the compression members

with hoops having 90ο hooks but it has been recommended by

seismic codes to provide 135ο

hooks for proper confinement.

The inferior performance was recorded in case of confinement

with 90ο hooks with regard to strength and ductility.

In this paper the change in strength of specimens with 90ο hooks

(non-seismic hooks) and 135ο hooks (seismic hooks) has been

studied keeping other factors as constant.

Material and Methods

Ordinary Portland cement (OPC) of 43 grade was used for the

production of concrete. The locally available river sand and

20mm basalt aggregates were used. TMT bars of fe415 grade of

diameter 5 mm, 6 mm and 8 mm were used. A nominal concrete

mix of M20 grade (1:1.67:3.34) as specified in IS 456:2000 was

used for this investigation. The constituents were batched by

weight and mixed manually with water-cement ratio of 0.55 was

used. Moulds of prism ratio of 2 i.e. 15cm diameter and 30 cm

height and 15cmx15cmx30cm size were prepared.

The two different geometry for all columns were used. The

columns had a square cross section 150×150 mm and length 300

mm and circular columns had a section of 150 mm diameter

with 300 mm height. The longitudinal reinforcements, ribbed

bars with a diameter of 8 mm- 4 nos. in square section and 6

mm- 6nos. in circular cross section were placed into the section.

The transversal reinforcement was formed by closed stirrups

with a diameter of 5 mm. The longitudinal distance between the

stirrups at the middle part of the columns was 25, 50, 75, 100,

125 and 150 mm (these dimensions are used in the subsequent

notation of the series, e.g. SS1 is a square normal strength

(M20) concrete column with a 1” (25 mm stirrup distance at the

mid height with seismic detailing), NS1 is a square normal

strength (M20) concrete column with a 1” (25 mm stirrup

distance at the mid height with non-seismic detailing) and C1 is

a circular section normal strength (M20) concrete column with a

1” (25 mm stirrup distance at the mid height). The distance of

the stirrups at the ends was denser to prevent damage in this

region caused by introducing the load and by possible

geometrical imperfections.

ISCA Journal of Engineering Sciences_______________________________________________________ ISCA J. Engineering Sci.

Vol. 1(1), 40-44, July (2012)


Figure-1 Figure-2

Failure due to poor confinement

Test specimens

Figure-3 Figure-4

Confinement in the form of hoops (ties) Moulds for specimens

ISCA Journal of Engineering Sciences_______________

Vol. 1(1), 40-44, July (2012)

International Science Congress Association

Results and Discussion

The behavior of all series was very similar. Almost all

specimens failed around the mid height. As an example, most

specimens after collapse can be seen in figure

collapse was initiated by concrete softening at the mid height,

accompanied by symmetric buckling of both reinforcing bars at

the compressed side of the cross section. The bars always

buckled between ties, as can be seen in figure

specimen the failure was localized at the middle part of the

specimen in a wedge shape.

Figure-5

Failure of Specimen

The transverse confinement pressure exerted on the concrete

core is directly related to the quantity of transverse

steels. Figure-7 shows the behavior of confined concrete. From

the test results, it is found that the increase in volumetric ratio of

the transverse steel can directly enhance the strength of the

confined concrete and also simultaneously improv

of the confined concrete. Moreover, it is seen that the effective

configuration of the transverse steel can exert a greater effect in

the concrete strength.

Square specimens with seismic detailing

specimen Peak load

Ton

Peak strength

Mpa

SS1 70 43.95

SS2 62 38.93

SS3 53 33.28

SS4 42 26.37

SS5 38 21.15

SS6 33 20.72

S 32 13.94

__________________________________________________


The behavior of all series was very similar. Almost all

specimens failed around the mid height. As an example, most

specimens after collapse can be seen in figure-5. Column

collapse was initiated by concrete softening at the mid height,

accompanied by symmetric buckling of both reinforcing bars at

the compressed side of the cross section. The bars always

buckled between ties, as can be seen in figure-5. In most of the

n the failure was localized at the middle part of the

The transverse confinement pressure exerted on the concrete

core is directly related to the quantity of transverse reinforcing

7 shows the behavior of confined concrete. From

the test results, it is found that the increase in volumetric ratio of

the transverse steel can directly enhance the strength of the

confined concrete and also simultaneously improve the ductility

of the confined concrete. Moreover, it is seen that the effective

configuration of the transverse steel can exert a greater effect in

Conclusion

The behavior of three series of reinforced concrete columns

with a square and circular cross section was investigated.

Normal strength (M20 grade) concrete was chosen and two

different types of detailing were used (seismic and non

The columns were axially loaded. The results are summarized in

table 1,2 and 3. From the test results following conclusions can

be made:

By increasing the ratio of transverse reinforcement (ratio of

volume of hoops to the volume of core) the peak strength of

concrete substantially increases. The variation is shown in

figure-6. The circular hoops are more effective than square

hoops. The strength of cylindrical columns are more than square

section columns. Giving the same volumetric ratio of transverse

steel, a better confinement effect can be expected with the more

effective configuration type of the transverse steel. Thus, it

follows that a method to quantify the configuration of transverse

steel as an important variable of confinement effect is in need.

This study serves to illustrate that the degree of confinement

provided by non-seismic details based on the use of 90

less with comparison of seismic details. Columns with non

seismic detailing have limited confinement action, lesser

strength enhancement and limited ductility.

Keeping other factors same, the column with seismic h

(135ο) has higher peak strength than the column with non

seismic hooks (90ο). Compression failure (crushing)

accompanied by concrete softening and steel buckling

developed in the columns. In most of the cases failure of

columns localized into the middle part, where a wedge

failure pattern developed in the concrete, together with buckling

of the reinforcement between the stirrups. The damage zone had

approximately the same dimensions for all tested series. The

peak strength of the columns increase

stirrups becomes smaller. This was observed for both seismic

and non-seismic detailing.

So the columns have to be properly designed, detailed and

constructed such that they perform as desired during strong

earthquakes. Properly designed and detailed confining

transverse reinforcements prevent buckling of longitudinal bars,

avoid shear failure and provide sufficient ductility.

Table-1

Square specimens with seismic detailing

Peak strength

Mpa

Confinement

spacing mm

Volumetric ratio of transverse reinforcement

steel to confined concrete core

43.95 25

38.93 50

33.28 75

26.37 100

21.15 125

20.72 150

13.94 No confinement

_______ ISCA J. Engineering Sci.

42

The behavior of three series of reinforced concrete columns

are and circular cross section was investigated.

Normal strength (M20 grade) concrete was chosen and two

different types of detailing were used (seismic and non-seismic).

The columns were axially loaded. The results are summarized in

the test results following conclusions can

By increasing the ratio of transverse reinforcement (ratio of

volume of hoops to the volume of core) the peak strength of

concrete substantially increases. The variation is shown in

r hoops are more effective than square

hoops. The strength of cylindrical columns are more than square

section columns. Giving the same volumetric ratio of transverse

steel, a better confinement effect can be expected with the more

type of the transverse steel. Thus, it

follows that a method to quantify the configuration of transverse

steel as an important variable of confinement effect is in need.

This study serves to illustrate that the degree of confinement

details based on the use of 900 hooks is

less with comparison of seismic details. Columns with non-

seismic detailing have limited confinement action, lesser

strength enhancement and limited ductility.

Keeping other factors same, the column with seismic hooks

) has higher peak strength than the column with non-

). Compression failure (crushing)

accompanied by concrete softening and steel buckling

developed in the columns. In most of the cases failure of

le part, where a wedge-shape

failure pattern developed in the concrete, together with buckling

of the reinforcement between the stirrups. The damage zone had

approximately the same dimensions for all tested series. The

peak strength of the columns increases as the distance between

stirrups becomes smaller. This was observed for both seismic

So the columns have to be properly designed, detailed and

constructed such that they perform as desired during strong

esigned and detailed confining

transverse reinforcements prevent buckling of longitudinal bars,

avoid shear failure and provide sufficient ductility.

Volumetric ratio of transverse reinforcement

steel to confined concrete core ρs

0.025

0.012

0.008

0.006

0.005

0.004

-

ISCA Journal of Engineering Sciences_______________

Vol. 1(1), 40-44, July (2012)


Square specimens with non

Specimen Peak load

Ton

NS1 63

NS2 54

NS3 46

NS4 40

NS5 35

NS6 30

Specimen Peak load

Ton

C1 71

C2 60

C3 51

C4 39

C5 33

C6 27

C 26

Acknowledgement

The authors would like to thank for the cooperation provided by

the Jabalpur Engineering Collage, Jabalpur. The work reported

in this paper is based on the efforts of the staff and faculties

from the Department of Civil Engineering of the J

Engineering Collage, Jabalpur. Their valuable contribution is

gratefully acknowledged.

__________________________________________________


Table-2

Square specimens with non-seismic detailing

Peak strength

Mpa

Confinement

spacing mm

Volumetric ratio of transverse

reinforcement steel to confined

39.55 25

33.90 50

28.86 75

25.11 100

21.97 125

18.84 150

Table-3

Circular specimens

Peak strength

Mpa

Confinement

spacing mm


reinforcement steel to

56.76 25

47.96 50

40.77 75

31.18 100

26.38 125

21.58 150

14.42 No confinement

Figure-6

Comparisons of three series

The authors would like to thank for the cooperation provided by

the Jabalpur Engineering Collage, Jabalpur. The work reported

in this paper is based on the efforts of the staff and faculties

from the Department of Civil Engineering of the Jabalpur

Engineering Collage, Jabalpur. Their valuable contribution is

References

1. Sheikh S.A. and Uzumeri S.M., Analytical Model for

Concrete Confinement in Tied Columns

Structural Engineering, 108(12),

2. Saatcioglu M. and Razvi S.R., High

Columns with Square Sections under Concentric

Compression, ASCE Journal of Structural Engineering,

124(12), 1438-1447 (1988)

_______ ISCA J. Engineering Sci.

43



concrete core ρs

0.025

0.012

0.008

0.006

0.005

0.004



concrete core ρs

0.025

0.012

0.008

0.006

0.005

0.004

-

Sheikh S.A. and Uzumeri S.M., Analytical Model for

Concrete Confinement in Tied Columns, ASCE Journal of

108(12), (1982)

tcioglu M. and Razvi S.R., High-Strength Concrete

Columns with Square Sections under Concentric

, ASCE Journal of Structural Engineering,

ISCA Journal of Engineering Sciences_______________________________________________________ ISCA J. Engineering Sci.

Vol. 1(1), 40-44, July (2012)


3. Cusson D. and Paultre P., High-Strength Concrete Col-

Lanns Confined by Rectangular Ties, ASCE Journal of

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8. www.iitk.ac.in/EQTip19.pdf (2012)

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10. IS 13920: 1993 (Reaffirmed 1998) Edition 1.2, Ductile

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World Conference on

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at Low and High Strain Rates, AC1 Structural Journal,

79(2), (1982)

13. Němeček J., Padevět P., Bittnar Z., Acta Polytechnica, 44

(5), 2004

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